Appropriate doses of 5-Bromodeoxyuridine (BrdU) have been shown to have the capacity to selectively and reversibly suppress cell differentiation functions in eukaryotes. This effect is exerted without significantly affecting the general viability and proliferative capacity of the cells. The rates of total DNA, RNA and protein synthesis are also unaffected. It is the purpose of this project to study the effects of BrdU upon the development of helper and suppressor T-cell function, and upon development of antibody-secreting capacity in B-cells. Other lymphocyte function characteristics, including the expression of cell surface antigens (HL-A), the development of killer-cell functions, and the production of soluble mediator factors will also be studied. The overall aim is the application of BrdU and related aspects to an analysis of lymphocyte differentiation programs.